Lab Report - Potassium permanganate and active Manganese dioxide as oxidants

Lab Report - Potassium Permanganate and Active Manganese Dioxide as Oxidants

Introduction to the Lab experiments

The oxidation of alcohols to carbonyl compounds is an important transformation in organic synthesis and because of its significant role, the development of newer oxidative protocols I continues to receive attention in spite of the availability of several methods to accomplish this objective (Murphy & Eulitz et al. 2001). The use of supported reagents 6 has gained popularity because of the improved selectivity, reactivity and associated ease of manipulation. Since only the polar reactants adsorbed on the surfaces of various mineral supports absorb microwave energy, a variety of reagents supported on such surfaces can be utilized for the enhancement of organic reactions using a simple microwave (MW) oven. Microwave-enhanced chemical reactions, especially on inorganic solid supports and that are conducted under solvenfless conditions, have attracted attention recently. They offer several advantages over the conventional homogeneous and heterogeneous reactions in view of the rapid reaction rates and higher yields of the ensuing pure products (Arthur, 1977).

Active manganese dioxide is a potentially useful oxidant and has been used for the oxidation of a variety of compounds. We report herein a facile and selective oxidation of alcohols to carbonyl compounds using silica-supported manganese dioxide (MnO2) under solvent-free conditions in a process that is accelerated by microwave irradiation; the drying of this reagent at elevated temperatures is known to enhance the activity.

Experimental Study

We examined various mineral supports such as clay, alumina, and silica and found that silica provides the best results in terms of formation of pure products. Also, the yields obtained are comparable to those reported for related reactions. In the absence of support, the isolation of products from the ensuing residues is rendered difficult and the oxidation is also accompanied by the formation of minor contaminants. The reagent, however, does not discrimm" ate between 1 ° and 2 ° alcohols as revealed by the oxidation of an equimolar mixture of benzTl alcohol and diphenylmethanol with MnO2 that is barely enough to oxidize one component. The procedure involves a simple mixing of neat alcohols with silica-supported MnO2 and irradiating the mixtures in a MW oven for 20-60 sec in the solid state.

Procedure

The oxidation of benzyl alcohol is representative of the general procedure employed. MnO2 'doped' silica (1.25 g, 5 mmol of MnO2 on silica gel, Selecto Scientific, 230-400 mesh with large surface area of 600m2/g) is thoroughly mixed with benzyl alcohol (108 mg, 1 retool) and the material is placed in an alumina bath inside the MW oven and irradiated for 20 seconds. Upon completion of the reaction, monitored on TLC (hexane: AcOEt, 10:1), the product is extracted into methylene chloride, solvent removed and the residue passed through a bed of silica gel (4 cm) to afford exclusively benzaldehyde. The overoxidation to carboxylic acid is not observed. The same reaction could be completed in 2 h at a comparable temperature of 55 °C in an oil bath ...